Transdimensional engineering

Transdimensional engineering was the technology of creating dimensionally transcendental objects, which the Fourth Doctor proclaimed a "key Time Lord discovery". It was used to make TARDISes bigger on the inside. The Doctor demonstrated how this could be done to Leela, by using two boxes of different size. He placed the larger box on the TARDIS console, and then walked to stand next to Leela and asked which one was bigger, which was pointed out by Leela, to which the Doctor stated it was smaller which was because of the distance. The Doctor said that if the large box was kept away at that distance, and had it nearby the larger box would fit inside the smaller box. Leela called that "silly," much to the Doctor's annoyance.(TV: The Robots of Death) The Thirteenth Doctor explained that the ability to engineer dimensions was beyond human comprehension. (TV: The Ghost Monument)